Visualizing and quantifying the electrical activity of brains, in particular at the cellular level, has facilitated the progress of research entailing the understanding and treatment of neurological diseases such as Alzheimer's and Parkinson's. Although electrode-based methods are valuable and traditional tools for measuring the membrane potential of single neurons and MRI imaging. These methods have provided scientists with knowledge of large-scale brain activity over time scales of seconds to minutes, but no method to date has elucidated the connection between microscopic interactions at the neuronal level and macroscopic structures that perform complex computations. Moreover, electrode-based methods suffer from their mechanically invasive nature and inability to target genetically labeled subpopulations, or to monitor subcellular compartments. Besides limited by its temporal resolution, MRI also requires a costly magnetic field to achieve high spatial resolution.
By comparison, optical imaging techniques are well-positioned for measuring membrane potential noninvasively on multiple spatial and temporal scales, for both subcellular compartments and neuronal microcircuits. Deep optical imaging at high resolution inside biological tissue is challenging to implement because of the strong scattering characteristics of biological tissues. One optical imaging technique, multiphoton microscopy (MPM) has extended the imaging depth of high-resolution optical imaging and has enabled visualization of neuronal activity in the brains of small animals, but light scattering by biological tissues limits the penetration depth of MPM to about 1 mm.
A need exists to measure membrane potential non-invasively using high resolution optical imaging methods capable of reaching deep tissues within the brain of an animal subject.